1981 Porsche 911 Targa - 8.2 Litre Mid-Engine V8


When I was 15 years old in 1972 I read about this type of engine transplant written up in "Peterson's Complete Book of Engine Swapping No. 2" and always thought is was a very clever idea. That writeup was for an Olds 425 V8 into a Porsche 912. That book had been sitting on my shelf for years and when I reached 45 the itch was just to great to resist and I decided to build my own.

This engine swap is actually very easy to do. I used a 1970 Cadillac Eldorado 500 cid engine and TurboHydramatic 425 transaxle. The assembly is just as Cadillac designed it. After removing the Porsche engine & transaxle it was a simple matter of building motor mounts, CV joint adaptors and axles, a custom radiator and exhaust system. The Cadillac system has a single front and dual rear mount setup that was easily adaptable to the stock Porsche mounts.

Fitting the package in

The Porsche shock tower crossmember and sheetmetal behind the rear seat had to be removed. I chose a Targa body because of the extra unibody rigidty provided to help when this sheetmetal was removed. The body is still surprisingly stiff. When I was building the car onlookers swore that the back window would pop out and the chassis would twist. But it is all still nice and stiff.

When the final drive & axles arealigned properly with the rear wheels the powerplant is located perfectly fore/aft.....almost like Dr. P and Cadillac had planned for this swap to happen. The engine fits low in the back seat area as can be seen in the photos.

The Engine

The engine is a 1970 Cadillac 500 cid block specially build by Maximum Torque Specialties of Wisconson. This is THE performance Cadillac engine builder. The engine has been slightly bored, decked, and fully balanced. Forged rods and pistons are used, the very expensive Bulldog aluminum heads, pretty healthy cam with roller rockers and specially designed valve train. Roller timing gear & chain, high flow oil pump, and lots of other goodies went into building this engine. It pulled 517 +/- hp on the dyno. It idles a little rough and gives this Porsche a very suspicious sound! To keep engine area temperatures low we ceramic costed the exhaust manifolds in and out to direct exhaust gas heat out the tailpipe. We used the stock manifolds to keep it quieter under the luggage (since the engine is literally right behind you). A high torque starter, remote oil filter, Holley spread-bore double pump carb, and lots of custom billet stuff was also installed.

When you pull that luggage off and your onlookers see those "Porsche 8.2 Litre" valve covers you'll grin with delight at their complete bewilderment.

The Transmssion

There are only a few people that know about these TurboHydrmatic 425 Automatic transmissions. They are essentially a TurboHydramatic 400 with reverse rotation and a different valve body. These transmission were used in GMC motorhomes and so there is tremendous strength there and great aftermarket parts are available. This transmission was built to heavy motorhome specs, has a Trans-Go Shift kit, and is the VERY SPECIAL Switch-Pitch torque convertor system. For those of you not familiar, applying a 12V signal to a transmission terminal changes the convertor from low stall to high stall. This gives great hi-stall off the line performance and exceptional pull with it switches back to low stall. For more info just email me. I also have an aftermarket Switch-Pitch transmission controller that I have not installed and does with the car.

Final Drive

I used a 2:73:1 final drive. I tried a 3:17 ratio but the engine just revved too high at cruising speeds. With the huge torque of this motor and the dual range torque convertor the 2:73 gears are great.

In terms of getting the power to the rear tires, I have used Tubro Carrera CV joints because they are stronger and also allow the greater axle shaft angles (because the axles are shorter then stock). Axles were custom built by Dutchman Motorsports in Portland, OR according to the torsional moment strength specs I provided. To carry the extra weight I've installed 30mm torsion bars. The ride is great (perhaps a touch stiff) and the car sits very nicely.

Please take note that the wheels are 7's in front and 8's in the rear. These are the largest size stock Porsche rims and tires that fit this car properly and all are brand new (and were expensive).

Powerplant Installation

After removing the shock tower and rear seat sheetmetal the engine just lifts up from below. I use the stock corner engine mounts in the back with different biscuits. The front mount was CAREFULY welded to the tosion bar tube.


I have had three exhaust systems and have settled on a MagnaFlow dual in, duul out muffler mounted in the typical Porshe location. 2.5" pipes are used. There is NO "humming" effect (Flowmasters sounded the worst). Certainly headers were considered BUT because the engine is so close to the interior I think the noise would be unpleasant. Besides....the power and torque is pretty insane anyway and I like the "sleeper" design.


Engine cooling is handled by a specially constucted aluminum dual cross flow radiator. This radiator also contains the transmission cooler and uses two VERY high capacity fans. A high capacity water pump was also used. Several months ago there were some concerns with cooling as the car was getting hot. After installation of a large oil cooler up front with an electric fan activated with the radiator fan the car is staying cool at all times in stop and go and light freeway driving here in Southern California in75 to 90 degree weather.


What everyone loves is the Hartman luggage engine cover. I purchased used Hartman luggage on ebay and my fantastic interior guys (Roberto's Auto Trim in Costa Mesa, CA) figured out how to cut, hollow and combined the luggage to make the cover. The inside of the cover uses marine "SoundStop" engine room instalation to keep the passenger compartment quiet and cool.

The tachometer was recalibrated for an eight cylinder pulse and the MSD tach adaptor makes it all work just great. The oil pressure, water temperature, and associated idiot lights all work as well. The speedometer is not hooked up yet.....I have a really clever commercially made adaptor that coverts the mechanical Hydramatic speedo drive to an electronic pulse.

Shifting is handled by a common Lokar automatic shifter and shifter cable. It mounts right where the Porsche shifter does and looks totally stock.

I've had the seats redone and all of the interior carpet replaced. The interior is near immaculate.

THERE IS NO HEATER. It can easily be done using a self contained aftermarket water heater (from Summit, Jeg's). I live in So. Cal and so there is typically no need.


This project turned out just terrific. The powerplant package fits just great, it's a total sleeper, and people just can't believe it can be true.

I am selling the car because my dream was to build one and I did it. I'm not a drag racer or a track guy. I just like to invent and build. The car is very nice to drive and watch out when you put the pedal down.

I had a hard time pricing this car. As you can imagine I've got a bit of money in it and a lot of time....but that time was very enjoyable as I planned and executed the design. I built this myself, mostly in my garage by myself, but of course had help with welding, machine work, radiator, etc. My reserve is probably a little high but I do think that a serious car guy will hat appreciates design and work it took to execute…..and wants something completely unique.